Method for producing porous metal sintered molded bodies

ABSTRACT

The invention relates to a method for producing porous metal sintered molded bodies, wherein expandable polymer particles, in which a sinterable metal powder is dispersed, are expanded to form a molded body. The molded body is subjected to a heat treatment, wherein the polymer is expelled and the sinterable metal powder is sintered to form a porous metal sintered molded body. Preferably, styrol polymers are used. The sinterable metal powder is selected, for example, from aluminum, iron, copper, nickel, and titanium.

The invention relates to a process for producing porous sintered shapedmetal bodies.

Metallic foams have some interesting properties: compared to the solidmetal, their density is greatly reduced. However, they still have a highspecific stiffness and strength. In the case of an impact, the cellularstructure converts a great deal of kinetic energy into deformationenergy and heat, so that metallic foams are well suited to incorporationin crash elements. Compared to polymer foams, metal foams have asignificantly higher strength and heat resistance. Further potentialapplications include heat shields, filters, catalyst supports,sound-absorbing cladding or the production of very light, foam-filledrollers for the printing or paper industry.

Metal foams can be produced in various ways. In the “powder route”, ametal powder and a pulverulent blowing agent, e.g. titanium hydridepowder (TiH₃) are mixed, processed by uniaxial pressing or extrusion toform a foamable semifinished part and heated to the melting point of themetal. The blowing agent liberates gases and foams the molten metal.Depending on the time for which the temperature is held, the foamstructure such as the relative density, pore size and the like changes.In the “melt route”, gas is blown into a metal melt and the foamed metalsolidifies. To stabilize the bubbles in the melt, it is possible to add,for example, SiC particles to the melt.

Shaped metallic bodies can be produced by injection molding ofthermoplastic compositions comprising metal powders together with apolymer as organic binder. These are highly filled organic polymermolding compositions. After injection molding, extrusion or pressing ofthe thermoplastic composition to form a green body, the organic binderis removed and the binder-free green body obtained is sintered. Porousshaped metallic bodies can also be obtained by concomitant use ofblowing agents.

Thus, WO 2004/067476 discloses a process for producing a cellularsintered shaped body, in which metal powder is mixed with bindercomponents and expandable polystyrene particles (EPS) are incorporatedas blowing agent. This thermoplastically flowable molding composition isintroduced into a housing mold for expansion of the molding composition,converted into a molten state and foamed. The foamed molding compositionis solidified, the organic components are removed and the shaped bodywhich has been treated in this way is sintered. The foaming step shouldoccur with formation of individual expanded polystyrene foam particleswhich each take up a closed three-dimensional space in the moldingcomposition and have a narrow diameter distribution. In this process,binder and foamable material are necessarily different from one another.Production of the molding composition is complicated and requiresnumerous successive steps.

According to WO 2004/067476, shaped bodies having simple geometries areobtained by pressing of a pulverulent EPS-comprising molding compositionto form pressed bodies and subsequent foaming by means of steam in aperforated mold. Geometrically complex moldings are said to beobtainable by shaping and foaming of the molding composition by means ofknown injection molding processes. The process has the disadvantage thatthe pores produced by the EPS particles are very large, as a result ofwhich only few stabilizing struts remain in the shaped body and theseare also inhomogeneously distributed. Finally, materials whosemechanical properties are unsatisfactory for many applications areobtained.

DE 103 28 047 B3 describes a process for producing a component composedof metal foam, in which a plurality of metal foam building blocks whichcan be obtained by introduction of energy into and at least partialfoaming of pellets comprising a metal powder and a blowing agent powder,e.g. a metal hydride, are arranged in three dimensions. The metal foambuilding blocks which have been arranged in this way are subjected to anafter-treatment so that adjacent metal foam building blocks are joinedto one another by positive locking, fusion and/or adhesively. Adisadvantage of this process is that in the production of the metal foambuilding blocks it is possible for a partial collapse of the metal foamformed to occur, leading to uncontrollable formation of denser zones inthe interior of a building block produced in this way and a lowreproduction accuracy. Without adhesive bonding, the individual metalfoam building blocks do not adhere to one another.

It is an object of the invention to provide a process for producingporous sintered shaped metal bodies, which is free of the abovedisadvantages.

The object is achieved by a process for producing porous sintered shapedmetal bodies, wherein expandable polymer particles in which a sinterablemetal powder is dispersed are foamed to form a shaped body and the greenshaped body is subjected to a heat treatment in which the polymer isdriven off and the sinterable metal powder sinters to give a poroussintered shaped metal body.

In an advantageous embodiment, the expandable polymer particles areintroduced into a mold which is closed, preferably on all sides, afterfilling and the expandable polymer particles are foamed, for example bytreatment with steam and/or hot air. The geometry (three-dimensionalshape) of the mold usually corresponds to the desired geometry of thefuture molding.

In most cases, preference is given to prefoaming the expandable polymerparticles before introduction into the mold. During prefoaming, theexpandable polymer particles are heated with mechanical agitation, e.g.by fluidization by means of a hot gas, in particular air and/or steam.Prefoamers which are suitable for this purpose are known to thoseskilled in the art from the production of EPS insulation materials.Temperatures of, for example, 60 to 120° C. are generally suitable.Under these conditions, the particles expand as a result of thevaporizing blowing agent and partially also as a result of the steamwhich has penetrated into them to form a closed-cell structure in theinterior of the bead. During prefoaming, the polymer particles do notfuse with one another and remain as discreet particles.

The density of the future shaped bodies can be influenced via the degreeof foaming which depends mainly on the duration of the heat treatment.The duration of the heat treatment in prefoaming is typically from 5 to100 seconds.

Without the prefoaming step, nonuniform expansion and filling of themold can occur during foaming of the expandable polymer particles in themold, with the expandable polymer particles in the vicinity of theheated walls of the mold expanding to a greater extent than particles inthe interior of the mold.

In general, the mold is only partly filled with the (optionallyprefoamed) expandable polymer particles. During foaming, the polymerparticles expand and positively fill the initially incompletely filledmold with foam. The polymer particles are fused to one another duringthis operation.

Particularly in the case of complicated geometries, it can also beadvantageous to keep the empty volume in the mold low and optionallycompact the bed of the (optionally prefoamed) expandable polymerparticles introduced into the mold and in this way eliminate undesirablevoids. Compaction can be achieved, for example, by shaking of the mold,tumbling motions or other suitable measures.

Foaming is usually effected by heating to, for example, from 60 to 120°C., preferably from 70 to 110° C., e.g. by heating the filled mold bymeans of steam, hot air, boiling water or another heat transfer medium.Foaming increases the volume of the polymer component of the particles,with the interstices in the bed being filled out by the expandingpolymer particles and a shape-producing assemblage of the individualparticles occurring as a result of force interactions between theparticles whose volumes are increasing. The polymer particles melt onthe mutual contact surfaces, so that the polymer particles fuse togetherto give a shaped body (green body). The mold proscribes the shape andvolume of the green body. The shaped body which has a sufficient greenstrength can be taken from the mold.

The pressure during foaming is usually not critical and is generallyfrom 0.05 to 2 bar. The duration of full foaming depends, inter alia, onthe size and geometry and also the desired density of the molding andcan vary within wide limits.

The process of the invention starts out from expandable polymerparticles in which a sinterable metal powder is dispersed. Theexpandable polymer particles are preferably free-flowing or flowreadily. The proportion by weight of the dispersed sinterable metalpowder, based on the total weight of polymer and sinterable metalpowder, is preferably from 60 to 95% by weight, in particular from 65 to90% by weight. In the polymer particles, the polymer forms a continuous(coherent) phase in which the sinterable metal powder is dispersed.

The expandable polymer particles preferably comprise a physical blowingagent such as aliphatic hydrocarbons having from 2 to 7 carbon atoms,alcohols, ketones, ethers, halogenated hydrocarbons, carbon dioxide orwater or mixtures thereof. Preference is given to isobutane, n-butane,isopentane or n-pentane or mixtures thereof. The expandable polymerparticles generally comprise from 2 to 20% by weight, preferably from 3to 15% by weight, of blowing agent, based on the polymer in theexpandable polymer particles. The blowing agent is present in theexpandable polymer particles as a molecular solution in the polymerand/or as included droplets.

The expandable polymer particles are preferably essentially spherical,but another shape such as rod-shaped or lens-shaped pellets is alsopossible. The expandable polymer particles generally have a diameter (orlength in the direction of the largest dimension in the case ofnonspherical particles) of from 0.5 to 30 mm, in particular from 0.7 to10 mm.

The expandable polymer particles can be obtained in various ways.

The expandable polymer particles can be obtained, for example, byproducing expandable thermoplastic polymer pellets by mixing a blowingagent and a sinterable metal powder into a polymer melt and pelletizingthe melt. The expandable polymer particles are preferably produced bymeans of an extrusion process. Here, the blowing agent is mixed into apolymer melt via an extruder, the sinterable metal powder is mixed inand the polymer melt is pushed through a die plate and pelletized togive particles. The melt is usually cooled after introduction of theblowing agent. Each of these steps can be carried out by means of theapparatuses or apparatus combinations known in plastics processing. Thepolymer melt can be taken directly from a polymerization reactor or beproduced in the mixing extruder or a separate melting extruder bymelting of polymer pellets. Static or dynamic mixers are suitable formixing in the blowing agent and the sinterable metal powder. Cooling ofthe melt can be carried out in the mixing apparatuses or in separatecoolers. Possible pelletization methods are, for example, pressurizedunderwater pelletization, pelletization using rotating knives andcooling by spray misting of cooling liquids or pelletization byatomization.

The sinterable metal powder is appropriately mixed in via a sideextruder. For example, a substream of the melt stream initially obtainedcan be branched off via a melt valve into a side stream before passagethrough the die plate. The metal powder is added to the side stream andmixed homogeneously into the melt stream. Finally, the main stream andthe additive-comprising side stream are mixed and discharged via the dieplate. To be able to meter the metal powder with sufficient accuracyinto the melt stream, the powder can be pasted beforehand. This meansthat it is incorporated into a liquid which is compatible with the meltand the metal powder so as to form a paste having a preferably highviscosity.

A suitable process is described, for example, in DE 10 358 786 A1, whichis hereby fully incorporated by reference.

If temperatures above the flash point of the metal powder are reachedduring production of the expandable polymer particles, a suitableprotective gas such as nitrogen or argon is preferably passed throughthe apparatuses.

As an alternative, pellets can firstly be produced by mixing asinterable metal powder into a polymer melt and pelletizing the melt.These pellets can then be reshaped into beads in aqueous suspension inheated and stirred pressure vessels at temperatures in the vicinity ofthe softening point and at the same time impregnated with blowing agent.This conversion into beads gives bead-shaped particles having a definedparticle size. The conversion into beads is generally carried out atfrom 120 to 160° C., e.g. about 140° C., over a period of from 1 to 24hours, e.g. from 12 to 16 hours. Suitable processes are described, forexample, in DE-A 25 34 833, DE-A 26 21 448, EP-A 53 333 and EP-6 95 109,which are fully incorporated by reference.

As an alternative, the pellets can be impregnated with blowing agentunder superatmospheric pressure at a temperature below the softeningtemperature of the polymer. A pressure of from 25 to 70 bar (absolute),e.g. about 50 bar, is suitable for this purpose. The temperature can be,for example, from 25 to 60° C., e.g. about 40° C. A time of from 0.5 to20 hours, e.g. about 8 hours, is generally suitable. For this purpose, apressure-rated apparatus, e.g. an autoclave, is charged with thepellets, the blowing agent is added in such an amount that it preferablycompletely covers the pellets and the apparatus is closed. Air isdisplaced by an inert gas such as nitrogen. The apparatus is then heatedand the desired pressure is set. The pressure is established asautogenous pressure of the blowing agent at the treatment temperature oris set by injection of inert gas.

As sinterable metal powders, mention may be made of, for example,aluminum, iron, in particular iron carbonyl powder, cobalt, copper,nickel, silicon, titanium and tungsten, among which aluminum, iron,copper, nickel and titanium are preferred. As pulverulent metal alloys,mention may be made by way of example of high- or low-alloy steels andalso metal alloys based on aluminum, iron, titanium, copper, nickel,cobalt or tungsten. Here, it is possible to use either powders offinished alloys or powder mixtures of the individual alloy constituents.The metal powders, metal alloy powders and metal carbonyl powders canalso be used in admixture. When mixed metal powders are used, themelting points of the components of the mixture should not differ toomuch from one another, since otherwise the lower-melting component flowsand the higher-melting component remains. The maximum melting pointdifference is preferably 800° C. or less, in particular 500° C. or lessand most preferably 300° C. or less.

Suitable metal powders are, for example, atomized metal powders whichhave been obtained by spraying of liquid metal with compressed gases.

Carbonyl iron powder is preferred as metal powder. Carbon iron powder isan iron powder which is produced by thermal decomposition of ironcarbonyl compounds. To maintain flowability and to preventagglomeration, it can, for example, be coated with SiO₂. Iron phosphidepowder can preferably be concomitantly used as corrosion inhibitor.Carbonyl iron powder has a small and uniform particle size; theparticles have an essentially spherical shape. The melt viscosity of thecomposites with polymers is therefore very low and the melting point isuniform. Suitable carbonyl iron powders are described, for example, inDE 10 2005 062 028.

Further preferred metal powders are powders composed of aluminum andcopper.

The particle sizes of the powders are preferably from 0.1 to 80 μm,particularly preferably from 1.0 to 50 μm.

Suitable polymers are thermoplastic polymers having a good uptakecapacity for blowing agent, for example styrene polymers, polyamides(PA), polyolefins such as polypropylene (PP), polyethylene (PE) orpolyethylene-propylene copolymers, polyacrylates such as polymethylmethacrylate (PMMA), polycarbonate (PC), polyesters such as polyethyleneterephthalate (PET) or polybutylene terephthalate (PBT), polyethersulfones (PES), polyether ketones or polyether sulfides (PES) ormixtures thereof. Particular preference is given to using styrenepolymers.

As styrene polymers, preference is given to using clear, colorlesspolystyrene (GPPS), high-impact polystyrene (HIPS), anionicallypolymerized polystyrene or high-impact polystyrene (A-IPS),styrene-α-methstyrene copolymers, acrylonitrile-butadiene-styrenepolymers (ABS), styrene-acrylonitrile (SAN),acrylonitrile-styrene-acrylic ester (ASA), methylacrylate-butadiene-styrene (MBS), methylmethacrylate-acrylonitrile-butadiene-styrene (MABS) polymers or mixturesthereof or with polyphenylene ether (PPE).

To achieve better dispersion of the particles in the polymer melt,dispersants can optionally be added. Examples are oligomericpolyethylene oxide having an average molecular weight from 200 to 600,stearic acid, stearamide, hydroxystearic acid, magnesium, calcium orzinc stearate, fatty alcohols, ethoxylated fatty alcohols, fatty alcoholsulfonates, ethoxylated glycerides and block copolymers of ethyleneoxide and propylene oxide, and also polyisobutylene.

The polymer is driven off by means of a heat treatment. The sinterablemetal powder is sintered to give a porous sintered shaped body. The term“drive off” comprises upstream decomposition and/or pyrolysis steps. Theheat treatment can be carried out in a single-stage or multistageprocess. Preference is given to driving off the polymer (binder removal)at a first temperature in a first step and sintering the resultingbinder-free shaped body at a second temperature. The second temperatureis generally at least 100° C. higher than the first temperature. If theshaped body is exposed directly to the sintering temperature, severesoot formation on the shaped metal body is frequently observed,presumably due to excessively rapid pyrolysis.

Binder removal and the sintering process can be carried out in the sameapparatus; however, different apparatuses can also be used. Suitablefurnaces for carrying out binder removal and/or sintering are convectionbox furnaces, shaft retort furnaces, convection shuttle kilns, hood-typefurnaces, elevator furnaces, muffle furnaces and tube furnaces. Beltfurnaces, combi-chamber furnaces or shuttle kilns are suitable forcarrying out the steps of binder removal and sintering in the sameapparatus. The furnaces can be provided with facilities for setting adefined binder removal atmosphere and/or sintering atmosphere.

To carry out binder removal, the shaped body is preferably exposedsuddenly to the binder removal temperature and not heated slowly to thebinder removal temperature, since otherwise the polymer can run and thefoam structure can be lost. It is thus generally not a good idea toleave the shaped body in the heating zone of the furnace during theheating phase. To carry out binder removal in the laboratory, it ispossible to use, for example, a tube furnace having a long interior tubeand to position the specimen in the tube but outside the heating zoneduring the heating phase. As soon as the target temperature has beenreached, the specimen can be pushed into the heating zone. Binderremoval can be carried out industrially using, for example, beltfurnaces in particular.

Binder removal is preferably carried out in a defined atmosphere. Ingeneral, preference is given to an inert atmosphere or reducingatmosphere, with a reducing atmosphere being particularly preferred. Inthe case of metals such as aluminum, zinc or copper, it can beadvantageous to carry out binder removal under slightly oxidizingconditions in order to increase the green strength. Better removal ofresidual carbon and a strength-increasing oxide skin on the surface ofthe metal powder particles are achieved in this way.

A temperature of from 150 to 800° C. is generally suitable for binderremoval. In the case of iron, a temperature of about 700° C. has beenfound to be useful, and a temperature of from 400 to 600° C. has beenfound to be useful in the case of aluminum. The duration depends greatlyon the size of the shaped bodies.

Binder removal is followed by a sintering process. This sinteringprocess can be carried out at a temperature of from 250 to 1500° C. Inthe case of iron, a sintering temperature of from 900 to 1100° C. hasbeen found to be useful, and a temperature of not more than 650° C. hasbeen found to be useful in the case of aluminum. The sinteringatmosphere can be matched to the metal used. In general, an inertatmosphere or reducing atmosphere is preferred, with a reducingatmosphere being particularly preferred.

As reducing atmosphere during binder removal and/or sintering, hydrogenor mixtures of hydrogen and inert gas, e.g. a hydrogen/nitrogen mixture,have been found to be useful. The mixture of hydrogen and inert gaspreferably comprises at least 3% by volume of hydrogen.

The shaped body can sometimes undergo “after-foaming” during pyrolysis.It can be advantageous to carry out the pyrolysis in a mold havingperforated walls, with greater filling of the mold and also compactionand conglutination taking place.

High-strength porous metallic light-weight bodies are achieved accordingto the invention.

The invention is illustrated by the following examples.

Example 1

a) Extrusion of polystyrene with carbonyl iron powder:

4.0 kg of polystyrene (obtainable under the designation 158K from BASFSE, Ludwigshafen, Germany) were compounded with 16 kg of carbonyl ironpowder (carbonyl iron powder EQ, obtainable from BASF SE) in an extruderand the melt was pelletized by die-face pelletization to give pelletshaving an average particle size of about 3 mm.

b) Pressure impregnation with pentane:

The pellets were then immersed in pentane S (80% of n-pentane, 20% ofisopentane) and maintained at a pressure of 50 bar and a temperature of40° C. in a pressure autoclave for 4 hours. This gave polymer particlesloaded with about 5% by weight of pentane.

c) Production of the green body:

The pellets were introduced into a closed cube-shaped steel mold havingan edge length of 4 cm and the mold was heated to about 100° C. by meansof steam for 10 minutes. The polymer particles expanded during thistreatment and fused to give a green body which was taken from the mold.

d) Binder removal and sintering:

The green body was sawn into smaller cubes by means of a saw andsubsequently placed in a porcelain boat in a fused silica tube. Thefused silica tube was installed horizontally in a hingedhigh-temperature tube furnace (model LOBA 11-50 from HTM Reetz, Berlin,Germany). The fused silica tube projected out of the furnace at bothends. The porcelain boat was firstly placed in an outer end of the fusedsilica tube, i.e. outside the heating zone. Nitrogen was passed throughthe fused silica tube.

The furnace was set to 700° C. As soon as the furnace reached atemperature of 700° C., the nitrogen flow was reduced by 50% from 20 l/hto 10 l/h and supplemented by a hydrogen flow of 10 l/h. The porcelainboat was subsequently pushed inside the fused silica tube into themiddle of the furnace. After the specimen had reached a temperature of700° C., it was left in the heating zone for 10 minutes. It was thenpulled from the heating zone again to the end of the fused silica tube.

The furnace was then set to 900° C. As soon as the set temperature hadbeen reached, the porcelain boat was pushed back into the middle of thefurnace. After a sintering temperature of 900° C. had been reached, thespecimen was left in the heating zone for 15 minutes. The porcelain boatwas then pulled out of the heating zone again and the furnace wasswitched off. After cooling, the specimen was taken out.

e) Examination of the mechanical properties:

The examination of the mechanical properties was carried out by a methodbased on the test standard DIN EN 826—compressive strength of insulationmaterials. Here, the compressive stress at 10-100% deformation and alsothe E modulus can be determined. Specimens having the same compositionwhich had all been subjected to binder removal at 700° C. for 10 minutesbut had been sintered at different temperatures (900° C. and 1000° C.)for different residence times were examined.

The following results were obtained:

Compres- Binder sive stress re- Sin- at 10% moval tering Max. defor- EExper- [° C./ [° C./ Density stress mation modulus iment min] min](g/cm³) (kPa) (kPa) (kPa) 1 700/10 900/5  1.27   4187   2527    370285 2700/10 900/10 1.43   6180   3397    452462 3 700/10 900/15 1.64  10654  6799    668253 4 700/10 900/30 1.59 11 746   9033 1 211 838 5 700/10900/60 2.29 14 794 12 247 2 633 667 6 700/10 1000/15  2.39 21 709 18 8834 777 498 7 700/10 900/30 1.98 20 179 17 348 3 608 267 1000/5  8 700/10900/15 2.28 26 571 19 246 5 103 210 1000/15 

Example 2

Kneading of polystyrene with carbonyl iron powder:

70 g of polystyrene 158K (from BASF SE, Ludwigshafen, Germany) weremelted in a kneader (model Messkneter H60 from IKA Staufen, Germany).280 g of carbonyl iron powder EQ were subsequently added a little at atime. The mixture was subsequently kneaded for 30 minutes. Afterkneading, the product was discharged and roughly pelletized. The coarsepellets were subsequently milled to an average diameter of about 5 mm ina mill. The further steps were carried out in a manner analogous toexample 1.

Example 3

Kneading of polystyrene with aluminum:

200 g of polystyrene 158K (from BASF, Ludwigshafen, Germany) were meltedin a kneader (from Linden, Marienheide, Germany). 622 g of coarsealuminum powder ASMEP123 CL (from ECKA, Fürth, Germany) weresubsequently added a little at a time and the mixture was subsequentlykneaded for 30 minutes. After kneading, the product was discharged androughly pelletized. The coarse pellets were subsequently milled to anaverage diameter of about 5 mm in a mill. The further steps were carriedout in a manner analogous to example 1, with binder removal andsintering being carried out in one step at 600° C. over a period of 5minutes.

Example 4

Kneading of polystyrene with copper

200 g of polystyrene 158K (from BASF, Ludwigshafen, Germany) were meltedin a kneader (from Linden, Marienheide, Germany). 910 g of copper RogalGK 0/50 (from ECKA, Fürth, Germany) were subsequently added a little ata time and the mixture was subsequently kneaded for 30 minutes. Afterkneading, the product was discharged and roughly pelletized. The coarsepellets were subsequently milled to an average diameter of about 5 mm ina mill. The further steps were carried out in a manner analogous toexample 1, with binder removal being carried out at 700° C. over aperiod of 5 minutes and sintering being carried out at 850° C. over aperiod of 10 minutes.

1-14. (canceled)
 15. A process for producing porous sintered shapedmetal bodies, wherein expandable polymer particles in which a sinterablemetal powder is dispersed are foamed to form a shaped body and theshaped body is subjected to a heat treatment in which the polymer isdriven off and the sinterable metal powder sinters to give a poroussintered shaped metal body.
 16. The process according to claim 15,wherein the expandable polymer particles are introduced into a mold andfoamed.
 17. The process according to claim 16, wherein the expandablepolymer particles are prefoamed before introduction into the mold. 18.The process according to claim 15, wherein the proportion by weight ofthe dispersed sinterable metal powder, based on the total weight ofpolymer and sinterable metal powder, is from 60 to 95% by weight. 19.The process according to claim 15, wherein the expandable polymerparticles comprise a physical blowing agent.
 20. The process accordingto claim 19, wherein the expandable polymer particles are obtained byimpregnating polymer particles in which the sinterable metal powder isdispersed with a blowing agent.
 21. The process according to claim 19,wherein the blowing agent is an aliphatic hydrocarbon or a halogenatedhydrocarbon.
 22. The process according to claim 21, wherein the blowingagent is pentane.
 23. The process according to claim 15, wherein thepolymer is a polymer or copolymer of styrene.
 24. The process accordingto claim 15, wherein the sinterable metal powder has an average particlesize of from 0.1 to 80 μm
 25. The process according to claim 15, whereinthe sinterable metal powder is aluminum, iron, copper, nickel ortitanium.
 26. The process according to claim 15, wherein the sinterablemetal powder is carbonyl iron powder.
 27. The process according to claim15, wherein the expandable polymer particles are essentially spherical.28. The process according to claim 15, wherein the expandable polymerparticles have a diameter of from 0.5 to 30 mm